Conventionally, there were a number of said installing tools (Refer to Patent Documents 1 to 6, for instance) wherein in a holding part of the retaining ring is provided a circular arc shaped notch being slightly larger than a half circle so that it does not abut an shaft of the retaining ring when it is fitted on said shaft, wherein not only grooves holding the retaining ring are formed in said notch, but also a slit is formed in the center area of the notch and the grooves in the vertical direction, wherein application of the opening of the retaining ring to the shaft and pressing of the installing tool, after inserting an outside diameter area of the retaining ring into the holding part so that the opening faces an end, slightly expand and open the retaining ring and then the holding part as well, which fits the retaining ring on the shaft.    [Patent Document 1] Unexamined Utility Model Publication No. 52-31100 (FIG. 1)    [Patent Document 2] Unexamined Patent Publication (Kokai) No. 53-148800 (Page 2, FIG. 1)    [Patent Document 3] Examined Utility Model Publication No. 6-16654 (Page 2, FIG. 1 to FIG. 2)    [Patent Document 4] Unexamined Utility Model Publication No. 7-40076 (Page 6, FIG. 2)    [Patent Document 5] Registered Utility Model Publication No. 3010240 (Pages 6–7, FIG. 10)    [Patent Document 6] Unexamined Patent Publication (Kokai) No. 11-28680 (Page 2, FIG. 4)
However, there was a problem to solve in the installing tool of the prior art. When it is fitted, the holding part at the end of the tool must open. Nevertheless, if it is made easily deformable, pushing force might deform it when it is fitted, and it could not be fitted. Thus, as it needs strength to some degree, resistance increases when it is pushed in, and more pushing force than necessary will be necessary, which thus makes the entire fitting work inefficient.